A single-mode oscillation that has a narrow spectral width is possible in a distributed feedback semiconductor laser in which a diffraction grating is provided along the optical axis.
In a gain-guided laser in which a stripe electrode is provided on a semiconductor stacked body including an active layer, the current confinement is insufficient; and the optical confinement effect in the lateral direction is small.
If the semiconductor stacked body that includes the active layer has a ridge cross section, current confinement is possible; and the optical confinement effect increases. However, the heat dissipation in the lateral direction from the active layer is insufficient.